What controls cooling fan on when ac is on?

I don't think AC works the way you think it does. There is no cooling fan in an AC system.. the operation of the engine fan (which draws air through the condenser and radiator) is controlled by a thermal switch, and its operation remains unchanged with the AC on. The blower motor which pushes air through the HVAC ducts is the same one used for both heat and AC, and is controlled typically by a switch.

AC cools by pressurizing and depressurizing the refrigerant and cycling it through the AC system, which causes it to absorb ambient heat and heat drawn from the passenger compartment.. it then cycles that heated refrigerant through the condenser, where the heat from the refrigerant is rapidly exhanged to the air passing through the condenser fins, causing a rapid cooling of the refrigerant.

To do this, certain pressures must be maintained in the system, and this is done by the compressor. Most automotive AC systems use a cycling clutch compressor, where the compressor clutch is engaged and disengaged through pressure switches in the system in order to regulate the system pressure.